This invention relates to short circuit transfer arc welding machines.
A conventional short circuit transfer arc welding machine is as shown in FIG. 1. In FIG. 1, reference numeral 1 designates a DC source circuit for converting alternating current into direct current; 13, a pulse waveform setting circuit for outputting a pulse current value instruction signal a; 14, a pulse period setting circuit for providing a pulse current which receives signals a and b from the setting circuits 13 and 14, to control the "on-off" period of a switching element 2 such as a transistor; 3, a DC reactor; 4, a flywheel diode for preventing the application of a reverse voltage (high voltage) due to the DC reactor 3 immediately after the switching element 2 is turned off; 5, an auxiliary power source for supplying a DC current (hereinafter referred to as a base current, when applicable) to maintain a welding arc; 6, a reel on which a welding wire 7 has been wound; a wire feeding motor for feeding the wire 7 towards a material to be welded, namely, a base metal 10; 9, a welding torch which the wire 7 penetrates; and 11, a welding current detector comprising a shunt resistor, etc. The output of the welding current detector 11 is applied to the switching instruction circuit 12, where it is compared with the pulse current value instruction signal a, so that the "on-off" timing of the switching element 2 is controlled to obtain a pulse current having a predetermined value.
The short circuit transfer welding operation of the welding machine thus constructed will now be described.
First, the pulse current period instruction signal b and the pulse current value instruction signal a which are selected with respect to a selected wire feeding speed are applied to the switch instruction circuit 12. As a result, the "on-off" instruction signal is applied to the switching element 2 by the switching instruction circuit 12, so that a pulse current having a constant value flows with a constant pulse period.
Thereafter, the wire electrode 7 is fed to the welding torch 9 from the reel 6, and the end 7a of the wire electrode 7 is brought into contact with the base metal 10. By the first pulse current, the end 7a of the wire electrode 7 is burnt off and an arc is produced between the wire electrode and the base metal, so that the end 7a of the wire electrode 7 and the base metal 10 are heated and made molten by the arc thus produced. At the same time, the welding wire 7 is fed so that the molten wire electrode 7 is pushed against the base metal 10. The wire electrode thus shorted with the base metal is burnt off by the next pulse current, so that an arc is produced again. The above-described operation is repeatedly carried out for welding.
In the above-described welding device, the wire electrode feeding speed is not associated with the period of generation of the pulse current; that is, the wire electrode feeding speed and the pulse current generating period are set separately. Therefore, sometimes the timing of pushing the molten wire end 7a against the base metal 10 is not synchronous with the timing of the application of the pulse current. If this occurs, the wire electrode 7 is caused to push the base metal for a relatively long period of time, as a result of which a large quantity of large sputters are scattered to stick to the base metal 10 or the components of the welding machine. The conventional welding machine is obviously disadvantageous for this reason.